The desirability of facing solar energy collectors and concentrators directly at the sun to receive direct solar radiation therefrom, or alternatively to follow the relative motion of the sun across the earth's sky with a reflector in order to direct light from the sun onto a solar energy collector or concentrator is well known. A significant problem has existed in arriving at a satisfactory drive system for such tracking operations, however. Tracking systems are necessarily operated by some motion producing means, and historically the energy required to effectuate the requisite motion of the tracking system has represented a very substantial portion of the solar energy collected in the system. Consequently, a principal function of conventional solar collectors and concentrators employing such tracking mechanisms has been to maintain their own operation, with the result being a minimal net output of energy. For this reason many commercially available solar energy collection systems have abandoned solar tracking as yielding insufficient benefits to compensate for the energy consumed in their own operation. Consequently, such conventional solar energy collection devices have been operated at substantially less than the efficiency of which they are capable.
Other solar energy collectors, especially those of the concentrator variety, have accepted the energy drain which results from the use of those solar tracking systems. Because of the extreme loss in efficiency which results from misalignment of concentrators, such as parabolic reflectors employing a solar energy collection conduit located at the parabolic focus, solar energy concentration systems have continued to employ tracking systems, despite the power consumed in their operation.
In the aforementioned U.S. patent application Ser. No. 744,290, a tracking system was described which operated upon an entirely different and unique principle and which requires far less power consumption than other conventional solar tracking systems. The tracking device described in the foregoing patent application discloses a solar tracking system in which a counterweight or other graviational influence is employed to aid the required movement of the collector to maintain an orientation directed at the sun. By balancing the collectors in this manner, tracking systems need only reposition a small counterweight and overcome static enertia of a collector rather than move the relatively large mass of the collector unaided. However, even though representing a substantial step forward in the art, the tracking system disclosed in U.S. Patent Application Ser. No. 744,290 still fails to deal with certain problems in positioning solar collectors using a tracking system.
Heretofore, tracking devices for following the course of relative movement of the sun across a field of view have attempted to maintain a solar energy receiving panel, plate or trough in precise alignment relative to the sun. However, in some solar collection systems, such precision of alignment is not warranted by the energy consumed in the process. This is especially true of flat plate or panel collectors which currently represent the most commercially significant form of solar energy collection devices. The orientation of a flat plate collector suffers very little from misalignment over a significant arc, up to 30.degree. in some systems. The efficiency of energy collection in most flat plate collectors varies only slightly between the hours of 10 AM and 2 PM when the collectors are positioned immobile facing the equator and oriented in a north south direction at an inclination to face the sun directly as it passes overhead.
It is an object of the present invention to provide a tracking system for solar energy collectors which will reorient the collector only when misalignment relative to the sun exceeds a predetermined adjustable limit of tolerance. The allowable levels of tolerance may be as much as 30.degree. with some flat plate tracking systems, or as little as 0.1.degree. when used with some types of solar energy concentrators. Normally, most adjustments of the tolerance limit will be in the range of 15.degree. to 2.degree.. Because of its flexiblity, the invention finds applicability for use with virtually any type of solar energy collector, since all collectors benefit from proper orientation toward the sun.
A further feature of the invention is that the realignment adjustments made are such as to minimize the number of times that the collector requires realignment during a solar day. Instead of merely reorienting the solar radiation receiving portion of the collector to a currently optimum position, the tracking system of the present invention instead directs the collector to a position slightly in advance of the current position of the sun. The amount by which the tracking mechanism "leads" the sun at the time of its reorientation is equal to the limit of tolerance prescribed. Thus, immediately following reorientation of the collector, the relative movement of the sun tends to bring the sun and the collector toward an optimum position of alignment. Once the sun has approached and reached the optimum position, it thereafter, continues in its relative movement and recedes from the optimum position of alignment until it is at a point of misalignment which is equal to the prescribed limit of tolerance in the direction of relative progression across the sky. By this time, the situation is reversed and the sun then "leads" the collector. This condition signals the requirement for a subsequent adjustment of the tracking mechanism so that the collector is again positioned to "lead" the sun.
Preferably, the tracking mechanism is operable between a pair of limit switches, one associated with an east facing orientation of the collector an the other associated with a west facing orientation. Realignment of the collector by the tracking mechanism continues throughout the course of a day until the west limit switch is reached. At this time, the sun is low enough on the horizon that further reorientation of the tracking mechanism is not warranted by the relatively small increase in efficiency that would result as contrasted with the power expended in moving the collector. Instead, actuation of the limit switch either immediately or upon the occurence of some other signal, such as the reduction of an ambient light level of the surrounding environment below a prescribed threshold, will trigger a return of the collector to an east facing position. In arriving at the east facing position, the east limit switch deactivates the tracking mechanism to prevent further movement until a threshold illumination level again prevails. This will not normally occur until the next morning, so that the tracking mechanism expends no power in reorienting the collector in darkness or upon extremely cloudy days.
The threshold light detection mechanism has uses other than just inhibiting reorientation of the collector from an east facing position on particularly cloudy days when the solar energy which could be collected does not warrant the expenditure of power in reorienting the collector. The threshold detector prevents a power expenditure in reorienting the collector during the course of cloudy periods during a day or when for any other reason a satisfactory illumination level is not achieved. This might occur by reason of coverage of the collector with dust, sand or snow.
A further feature of the invention is the manner of generation of the signals used to effectuate reorientation. Differential photosensors are employed in opposition to each other. Each photosensor is positioned for greater responsiveness to light impinging upon the collector from opposite directions at angles of incidence departing from 90.degree.. The photosensitive detectors provide separate outputs which are combined in opposition to provide a differential output signal. Moreover, the sum of the two outputs is also utilized so that the ratio between the difference and the sum of the two detector outputs may be derived. This ratio is employed as the controlling signal in effectuating reorientation of the collector. In so doing, the system compensates for different levels of ambient light, and thus maintains the particular tolerance limit established as being the most desirable. In the absence of control in this manner, the tracking mechanism would respond to extremely bright sunlit days by decreasing the allowable tolerance limit. Conversely, on somewhat cloudy days in which the ambient light level of the surrounding environment is still sufficient to achieve the requisite threshold, operation of the tracking system without compensating for ambient light level would result in an increase in the tolerance level. Either such variation is undesirable as such variations in tolerance limits do not correspond to relative efficiences of alignment under identical ambient light conditions, but rather are correlated only to the absolute levels of energy conversion.
A further object of the invention is to provide a tracking system with a safety mechanism which can respond to excessive operating temperatures. Typically, solar collectors employ a heat collection fluid circulating through a solar energy receiving station. When this fluid reaches a temperature above a predetermined safe limit, it is desirable to effectuate a temperature reduction. This is achieved in the present invention by actuating the tracking mechanism to direct the collector away from an orientation facing the sun to a position in which it does not face the sun. This allows the heat collection fluid to cool and thus avoid adverse effects to the solar collection equipment. Unsafe temperatures in solar collectors sometimes result when there is a blockage or leak in the solar collection fluid line. In such circumstances a "stall condition" exists in which no fluid is passing through the collector. Continued operation of the collector and of the pump which normally drives fluid through the collector can result in considerable damage to the equipment.
A further object of one embodiment of the invention is to provide an entirely self-contained tracking system for rotating a solar energy collector to face the sun by means of an electrically operated drive system in which no external electrical power source is provided. This is achieved by positioning photovoltaic cells on the collector. The electrical output of these cells may be utilized to operate the tracking system. The energy output of voltaic cells provides sufficient power for this purpose when the tracking system is constructed to draw a minimum of power. Accordingly, the provision of photovoltaic cells as a source of power for the tracking system, and indeed as a source of power to pump fluids circulating through one or more collectors, has particular application in connection with the tracking arrangement depicted in the aforesaid U.S. Patent Application Ser. No. 744,290 and in U.S. Patent Application Ser. No. 789,424, filed Apr. 21, 1977.